Ion guide with reduced noding effect

ABSTRACT

An ion optical arrangement (1) for use in a mass spectrometer comprises electrodes (11, 12, 14) comprising a multipole arrangement defining an ion optical axis, and a voltage source for providing voltages to the electrodes to produce electric fields. The ion optical arrangement is configured for producing a radio frequency electric focusing field for focusing ions on the ion optical axis. The radio frequency electric focusing field has a varying frequency so as to reduce any mass dependence of ion trajectories through the ion optical arrangement. The ion optical arrangement may further be configured for producing a static electric field in response to a DC bias voltage applied to the multipole arrangement. A superimposed varying electric field may be produced by superimposing an AC voltage upon the DC bias voltage.

FIELD OF THE INVENTION

The present invention relates to isotope ratio mass spectrometry (MS).In particular, the present invention relates to interference free, highresolution, multi-collector isotope ratio mass spectrometry andelemental analysis, for example in combination with a collision cell anda plasma source, such as an inductively coupled plasma (ICP) source.More in particular, the present invention relates to an ion guide with areduce noding effect.

BACKGROUND OF THE INVENTION

Multi-collector ICP-MS is an established method for high precision andaccurate isotope ratio analysis. Applications are in the field ofgeochronology, geochemistry, cosmochemistry, biogeochemistry,environmental sciences as well as in life sciences. Precise and accurateisotope ratio measurements very often provide the only information togain deeper insight into scientific questions which cannot be answeredby any other analytical technique. However, elemental and molecularinterferences in the mass spectrometer limit the attainable precisionand accuracy of the analysis.

These interferences are present in the sample material itself or aregenerated by sample preparation from a contamination source (usedchemicals, cleanliness of sample container, and fractionation duringsample purification) or are even generated in the ion source or in themass spectrometer. The problems with such interferences can be counteredby:

-   1. using a high mass resolution mass analyzer that discriminates    against interferences by detecting small differences in mass of the    interference relative to the sample ion;-   2. by sample preparation and chemical separation of interference    prior to mass analysis; and/or-   3. by using a collision cell integrated into the mass analyzer.    In a collision cell the chemical interferences are removed by    chemical reactions, and/or by kinetic energy discrimination, taking    advantage of different cross sections of molecular and elemental    species inside the pressurized collision cell which results in    different kinetic energy losses of molecular and elemental ions. By    means of a high pass energy filter following the collision cell the    lower energy molecular species can be discriminated.

A collision cell is an encapsulated volume within the ion optical beampath which is pressurized with a collision gas to cause interactions(i.e. collisions and/or chemical reactions between the ions and the gasmolecules). In order to generate efficient collisions and chemicalreactions inside the collision cell, the ions preferably are at a lowion beam energy of a few electronvolt (eV) only. The collision cellusually is a multipole ion guide which is powered by RF fields to guidethe ions through the collision cell. In order to achieve a reasonablegas pressure, the multipole ion guide is encapsulated in a compactvolume with small entrance and entrance apertures, typically in therange of 1-3 mm diameter. A collision cell coupled to a multi-collectormass spectrometer is disclosed in British patent application GB 2 546060 (Thermo Fisher Scientific (Bremen) & The University of Bristol).

Ions having different masses but the same energy travel at differentvelocities through the time dependent oscillating field of the collisioncell and as a result the ion trajectories are mass dependent. In otherwords, the trajectories depend on the mass of the ions traveling throughthe RF field. This effect is called “noding”. This can in particularpose a problem at the exit of the multipole structure, where ions ofdifferent masses may exit at different angles.

The mass dependence of the collision cell transmission can be a problemfor accurate isotope ratio measurements, even when it is small. However,for some analytical applications there is no other choice to removeisobar interferences but to use the collision cell.

For samples where no interferences are present it would be advantageousto avoid the low energy passage of the ions through the radio frequency(RF) multipole collision cell optics and to exclude any uncertainty ofthe discrimination effects caused in the collision cell (i.e. chemicaleffects as well as the noding effect).

It is noted that the undesired “noding effect” is not limited tocollision cells but may also occur in other ion optical arrangements,such as mass filters.

One way to solve this problem is to install a second beam path in themass spectrometer where the ion beam is deflected off axis prior to thecollision cell to bypass the collision cell and finally to deflect theions back onto the optical axis of the mass spectrometer. Such a dualpath ion optics arrangement is described in British patent applicationGB 2 535 754 (Nu Instruments). It allows to switch between the lowenergy collision cell beam path and an off axis static high energy beampath. This results into a rather complicated setup with several ion beamdeflectors causing image aberrations and alignment problems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ion opticalarrangement, such as a collision cell or a mass filter, for a massspectrometer which can largely avoid the noding problems related toexisting RF driven ion optics and which is simpler and more compact thanthe dual path arrangement of the prior art.

Accordingly, the present invention provides an ion optical arrangementfor use in a mass spectrometer comprising:

electrodes comprising a multipole arrangement defining an ion opticalaxis, and

a voltage source for providing voltages to the electrodes to produceelectric fields, wherein the ion optical arrangement is configured forproducing a radio frequency electric focusing field for focusing ions onthe ion optical axis, and

wherein the radio frequency electric focusing field has a varyingfrequency so as to reduce any mass dependence of ion trajectoriesthrough the ion optical arrangement.

By providing an ion optical arrangement or ion guide which has a varyingradio frequency (RF) electric field, in contrast to an RF electric fieldhaving a constant frequency, the noding effect and hence the massdependency of the ion trajectories through the ion optical arrangementis mitigated and may in some instances be completely eliminated. Varyingthe RF frequency changes the number of oscillations of the iontrajectory in the ion optical arrangement. In particular a cyclicallyvarying frequency is useful in mitigating the noding effect. The varyingRF frequency is therefore preferably a cyclically varying frequency.

The cyclical variation in the RF frequency is not limited to asinusoidal variation but may also comprise a saw-tooth shaped variationin which the frequency may linearly increase (or decrease) from a firstvalue to a second value, and then virtually instantaneously returns tothe original value. Other frequency variations can also be envisaged,such as square wave variations.

The frequency variations are preferably effected by variations in thevoltages supplied to the RF electrodes. When the ion optical arrangementcomprises a multipole arrangement, therefore, the voltage source may beconfigured for supplying a voltage having a varying frequency to themultipole arrangement so as to produce the radio frequency electricfocusing field having a varying frequency. The multipole arrangementcomprises at least four poles, preferably at least six poles. It hasbeen found that using a greater number of poles further reduces thenoding effect. Accordingly, an octupole arrangement may also beadvantageously used.

It is preferred that the varying frequency varies by at least 10%,preferably at least 20%. As will later be explained in more detail, ithas been found that the noding effect can be effectively reduced or eveneliminated by varying the number of oscillations of the ion trajectorywithin the ion optical arrangement. The number of oscillations can bechanged from, for example, 10 to a range of 9 to 11. By periodicallychanging the number of oscillations within this range, the massdependencies of the ion trajectories can be effectively reduced. Toachieve a range from 9 to 11 oscillations, as in this example, afrequency range of f₀±10% is preferred, where f₀ is the base RFfrequency. The base frequency is therefore, preferably cyclically,varied from f₀−10% to f₀+10%, resulting in a frequency range of 20% off₀. If a greater change in the number of oscillations is desired, agreater frequency swing will be required. For example, when the originalnumber of oscillations in the ion trajectories is 10, as in the exampleabove, a range from 8 to 12 oscillations may be achieved by using afrequency range of f₀−20% to f₀+20%.

It is noted that this advantageous change in the number of trajectoryoscillations can not only be achieved by varying the RF frequency, butalso by imposing a frequency upon a static electric field. Accordingly,the present invention also provides an ion optical arrangement for usein a mass spectrometer comprising:

electrodes comprising a multipole arrangement defining an ion opticalaxis, and

a voltage source for providing voltages to the electrodes to produceelectric fields, wherein the ion optical arrangement is configured forproducing a radio frequency electric focusing field for focusing ions onthe ion optical axis, wherein the ion optical arrangement is furtherconfigured for producing a static electric field, and wherein a varyingelectric field is superimposed upon the static electric field so as toreduce any mass dependence of ion trajectories through the ion opticalarrangement.

By superimposing a varying electric field upon the static electricfield, a similar reduction of the noding effect may be achieved as withvarying the RF frequency. It is noted that in some embodiments acombination of measures may be used, thus combining a varying RF fieldwith a varying field imposed upon the static field. By varying both theRF electric field and the static electric field, a further reduction ofthe noding effect may be achieved.

The static electric field may comprise a field produced by a DC biasvoltage applied to the multipole arrangement, the superimposed varyingelectric field being produced by superimposing an AC voltage upon the DCbias voltage. It is noted that the DC bias voltage may be positive ornegative. In some embodiments, the DC bias voltage may be equal to zero.If the DC bias voltage is originally non-zero, the superimposed AC(alternating current) voltage will typically not turn the DC biasvoltage in an AC bias voltage but in a DC bias voltage having a varyingamplitude, preferably a cyclically varying amplitude.

The static electric field may comprise an axial DC field produced by aDC auxiliary voltage applied to auxiliary electrodes arranged inparallel with the multipole, the superimposed varying electric fieldbeing produced by superimposing an AC voltage upon the DC auxiliaryvoltage. Such auxiliary electrodes may comprise so-called vanes whichmay be arranged in the spacings between the poles of a multipolearrangement. Such vanes are typically flat, elongate electrodes and mayserve to produce an auxiliary electric field, such as an axial dragfield. For such purposes, at least one auxiliary electrode may comprisea series arrangement of resistors for providing a voltage gradient inthe auxiliary electrode so as to produce an axial field gradient.

Other auxiliary electrodes may comprise ion lenses consisting of asingle electrode, such as an entrance electrode or an exit electrode, ora set of electrodes which together influence the ion beam. Instead of,or in addition to varying the frequency of the electric field, it ispossible to vary the energy and/or velocity of the ions. Thus, theelectric fields which are related to the energy and velocity of the ionsmay also be varied so as to counter the noding effect.

In the embodiments discussed above, RF electrodes, such as a multipolearrangement, are present. The invention is however not limited to ionoptical arrangements having a multipole arrangement and embodiments canbe envisaged in which no multipole arrangement is present. The presentinvention therefore also provides an ion optical arrangement for use ina mass spectrometer comprising:

electrodes defining an ion optical axis, and

a voltage source for providing voltages to the electrodes to produceelectric fields, wherein the ion optical arrangement is configured forproducing a static electric field, and

wherein a varying electric field is superimposed upon the staticelectric field so as to reduce any mass dependence of ion trajectoriesthrough the ion optical arrangement.

By superimposing a varying electric field upon the static electricfield, the same or similar advantages may be obtained. Embodiments ofthe present invention may therefore be summarized as an ion opticalarrangement for use in a mass spectrometer which is configured forvarying the number of oscillations of the ion trajectory within the ionoptical arrangement.

In embodiments without an RF field, the static electric field maycomprise an axial electric field, preferably an axial electric fieldhaving an axial field gradient. The static electric field may comprise afield produced by a DC bias voltage applied to an ion optical lens.

As mentioned above, the present invention is not limited to a collisioncell or a collision/reaction cell, but also provides other ion opticalarrangements. The ion optical arrangement according to the invention maytherefore comprise a mass filter.

The present invention also provides a mass spectrometer comprising anion optical arrangement as discussed above. The mass spectrometer of theinvention may further comprise at least one ion source, such as aninductively coupled plasma ion source, and at least one detectorarrangement, such as a multi-collector detector arrangement, andpreferably also a mass filter.

The present invention additionally provides a method of operating an ionoptical arrangement for use in a mass spectrometer, the ion opticalarrangement comprising:

electrodes comprising a multipole arrangement defining an ion opticalaxis, and

a voltage source for providing voltages to the electrodes to produceelectric fields, wherein the ion optical arrangement is configured forproducing a radio frequency electric focusing field for focusing ions onthe ion optical axis, the method comprising varying the frequency of theradio frequency electric focusing field so as to reduce any massdependence of ion trajectories through the ion optical arrangement.

The varying frequency may be a cyclically varying frequency. The varyingfrequency may vary by at least 10%, preferably at least 20%.

The method of the invention may further comprise supplying a DC biasvoltage to at least some electrodes and superimposing an AC voltage uponthe DC bias voltage. The static electric field may comprise an axial DCfield produced by a DC auxiliary voltage applied to auxiliary electrodesarranged in parallel with the multipole. The method may further comprisesuperimposing an AC voltage upon the DC auxiliary voltage to produce thesuperimposed varying electric field.

In another embodiment, the invention provides a method of operating anion optical arrangement for use in a mass spectrometer, the ion opticalarrangement comprising:

electrodes defining an ion optical axis, and

a voltage source for providing voltages to the electrodes to produceelectric fields, wherein the ion optical arrangement is configured forproducing a static electric field, and wherein the method comprisessuperimposing a varying electric field upon the static electric field soas to reduce any mass dependence of ion trajectories through the ionoptical arrangement.

Embodiments of the method according to the invention may comprisedetermining the number of oscillations of an ion beam passing throughthe ion optical arrangement over the length of the ion opticalarrangement, and varying the frequency of the radio frequency electricfocusing field and/or setting the frequency of the varying electricfield superimposed upon the static electric field such that the numberof oscillations is changed by at least one. By determining the number ofoscillations in the beam, it is possible to accurately determine theminimum required percentage of frequency change. The number ofoscillations may be determined by conventional methods, for examplevisually.

It is noted that the ion optical axis along which ions pass through theion optical arrangement may be straight but that this is not essential.In some embodiments, the ion optical axis through the collision cell isstraight but the path of the ions through the ion optical arrangementmay not be straight and may be partially or entirely curved, as in thearrangement of GB 2 546 060, for example.

The ion optical arrangement according to the invention may furthercomprise a pump for pressurizing the ion optical arrangement at leastduring the first operation mode in which it is used as a collision celland a pressure release mechanism for releasing gas pressure whenswitching from the first operation mode to the second operation mode.The pressurizing pump may be switched off in the second operation mode.In some embodiments, the pump may be reversed in the second operationmode. In an embodiment, the ion optical arrangement may comprise aswitchable pumping cross section in the collision cell housing forestablishing a higher gas pressure inside the first operation mode (lowcross section) and pumping the collision cell efficiently in the secondoperation mode (high cross section). The first operation mode may be alow energy mode while the second operation mode may be a high energymode. That is, the ions passing through the collision cell may have arelatively low energy in the first operation mode when gas is presentand a relatively high energy in the second operation mode, whenvirtually no gas is present.

Various pressure release mechanisms may be used. In an embodiment, thepressure release mechanism may comprise a valve operated by a Bourdontube so as to pneumatically operate the pressure release mechanism. ABourdon tube typically consists of a rounded or wound tube whichstraightens when inflated. In another embodiment, the pressure releasemechanism may comprise a relay so as to electrically operate themechanism. In some embodiments, a Bourdon tube and a relay mayadvantageously be combined.

In an embodiment, the pressure release mechanism comprises anelectrostatic mechanism which also allows to electrically operate themechanism. The electrostatic mechanism preferably comprises aninsulating foil provided with a conducting layer, which insulating foilcovers at least one opening in the collision cell when a first voltageis applied and is spaced apart from the at least one opening when asecond voltage is applied. Thus, the insulating foil can be movedtowards and away from openings in the housing of the collision cell byapplying suitable voltages.

The present invention yet further comprises a software program productfor carrying out the method described above, in particular for causing aprocessor to control the voltage source to produce suitable supplyvoltages to the electrodes. The software program product may comprise atangible carrier on which instructions are stored which allow aprocessor to carry out steps of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a multipole arrangement where noding occurs.

FIGS. 2A & 2B schematically show the noding effect in an ion opticalarrangement.

FIG. 3A schematically shows a partitioned multipole arrangement used inRF mode.

FIG. 3B schematically shows the partitioned multipole arrangement ofFIG. 3A used in DC mode.

FIG. 3C schematically shows the electric field near the ends of thepoles of FIG. 3A.

FIG. 4 schematically shows a multipole collision/reaction cell in whichthe invention may be utilized.

FIGS. 5A & 5B schematically show an embodiment of a pneumatic pressurerelease mechanism which may be used with the collision/reaction cell ofFIG. 4.

FIGS. 6A-6C schematically show an embodiment of an electrostaticpressure release mechanism which may be used with the collision/reactioncell of FIG. 4.

FIG. 7 schematically shows a mass spectrometer comprising an ion opticalarrangement in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned above, it is an object of the present invention to reducethe noding effect in an ion optical arrangement, such as acollision/reaction cell or a mass filter.

In GB 2 546 060, which is herewith incorporated by reference in thisdocument, the concept of a pre-mass filtered collision cell interfacedto a multi-collector mass spectrometer is disclosed. An RF quadrupolepre-mass filter is used which also introduces noding effects similar tothe RF collision cell. In the collision cell the ion beam trajectoriesare altered by the collisions and the strong phase correlation to theoscillating RF field of the ions traveling through the quadrupole isdisturbed by the collisions and thus leads to less mass dependenttransmission effects.

The small dependence of the trajectories depending on the mass is knownas “noding”. It is a result of the spatial oscillations of the ionsinside a multipole. Depending on the number of oscillations of the ions,they leave the cell with an angle/position vector that is massdependent. This effect can be amplified by the tuning parameters such asthe potentials of the entry and exit lenses which determine the inputand exit parameters of the ion beam entering and exiting the collisioncell. The DC bias potential of the multipole rods also determines thetravel velocity of the ions through the collision cell and has aninfluence on the noding.

By using higher order multipoles, from quadrupoles (4 poles) tohexapoles (6 poles) or octupoles (8 rods or poles), higher orderoscillations are added to the ion trajectories which make the massdependent differences of the trajectories less pronounced and which atthe same time increase the acceptance input aperture of the collisioncell. However, this beneficial effect is limited.

The pressurization of a collision cell by introducing a collision gas(e.g. helium) flow in the order of several ml/min results in multiplecollisions of the ions with the collision gas, which in turn results inscattering and random movements of the ions. These scattering eventsfurther reduce the phase correlation of the ion beam trajectories to theoscillating RF field and thus reduce the noding effect. The morecollisions the ions undergo the smaller the noding effect is. Especiallyfor heavier ions multiple collisions result into both a reduction of thekinetic energy and a reduction of the energy spread of the ions, whichimproves the focusing conditions and which is known as collisionalfocusing.

The momentum transfer per collision becomes more efficient the more thedifference in mass between both collision partners is reduced and mighteven stop the movement of the ions. For lighter masses approaching thelow mass range of helium (He), the overall transmission efficiencythrough the pressurized collision cell is significantly reduced. Thiscan partly be compensated by adding an axial electrical field gradientto the cell potential that actively drags ions from the entry to theexit aperture and therefore allows for an increased number of collisionsas well as for higher transmission for lighter ions.

The noding effect can be reduced by using higher order multipoles withhigh gas pressures and axial fields, but it cannot be eliminatedcompletely. Accurate and precise isotope ratio measurements usingcollision cells requires the availability of a calibrated standard andextensive calibration procedures. Tuning parameters need to becontrolled carefully.

The present invention provides a solution to the noding problem byproviding a collision/reaction cell that varies the number ofoscillations the ions undergo in the RF field. That can be done by:

-   1. a variation of the RF frequency, and/or-   2. a variation of the ions' energy/velocity in axial direction,    and/or-   3. any other lens element that influences the ion velocity.    The variation of the axial energy can be achieved by, for example,    superimposing an oscillation on the rod bias voltage (DC potential    of the rods that defines the energy the ions have in the multipole)    and/or by applying an oscillating voltage to the vanes (which may    also be referred to as drag electrodes in some embodiments, see    FIGS. 4A & 4B).

The amplitude of the applied variation is preferably such that thenumber of oscillations which the ions undergo changes by at least 1 overthe length of the collision cell. As mentioned above, the number ofoscillations n is given by the frequency and the velocity of an ion:

$n = {f \cdot l \cdot \sqrt{\frac{m}{2E}}}$

With:

f=frequency,l=length of multipole,m=ion mass, andE=ion energy.

In an embodiment, the minimum number of oscillations is in the order of10 (f=1 MHz, l=100 mm, m=7 amu and E=5 eV). Hence the frequencyvariation should preferably be at least 10 percent (it is noted that thenumber of oscillations n is directly proportional to f) or the energyvariation should preferably be at least 20 percent.

Accordingly, the invention provides at least the following advantages:

-   -   The use of RF frequency variations and/or superimposed        frequencies reduces noding effects.    -   There is only one optical axis in the system (no bypass optical        axis). This allows a compact geometry and reduced aberrations.    -   Since there is only one ion optical axis, the tuning of the        system is much easier compared to a complicated deflection setup        where the ion beam has to be steered along a bent bypass axis to        circumvent the collision cell or vice versa.    -   The principle of a segmented multipole lens also can be applied        to a quadrupole mass filter lens. This allows the ion optical        instrument to be switched from a low energy front-end RF        multipole lens design to a high energy DC lens design without        any noding effects.

As mentioned above, a problem that may arise in a multipole arrangementis noding. This effect is illustrated in FIG. 1. A multipolearrangement, which may be part of a collision cell 1 or of a massfilter, comprises rods or poles 11, to which an RF voltage may beapplied. An entrance electrode (front plate) 12 is provided with anentrance opening 13 for letting an ion beam 113 enter the multipolearrangement. An exit electrode (back plate) 14 is provided with an exitopening 15 for letting the (modified) ion beam IB′ exit the multipolearrangement.

As can be seen, some ions follow slightly different trajectories,resulting in the modified ion beam IB′. While the original ion beam 113was substantially uniform, the ion beam IB′ exiting the multipolearrangement is no longer uniform, different ions exiting at slightlydifferent angles. The trajectories shown in FIG. 1 are of ions havingthe same energy but different masses. Since different masses followdifferent trajectories, the probability that ions pass through the exitopening 15 (instead of hitting the end plate 14) is also mass dependent.In addition, the focusing of the ions emerging from the multipolearrangement in a subsequent ion optical device (such as a mass analyzer)may also become mass dependent. It will be clear that this isundesirable. In embodiments of the invention, therefore, the RFfrequency of the voltage supplied to the rods is varied. That is, the RFfrequency is not kept constant but is changed over time. Frequencychanges of at least 10% are preferred, although smaller frequencychanges such as 5% may in some embodiment also be used, also dependingon the length of the multipole arrangement. Frequency changes of 15% or20% may, however, be more effective in some multipole arrangements. Thatis, at an RF frequency of 1 MHz, for example, the frequency ispreferably made to vary at least from 0.90 MHz to 1.10 MHz (−10% and+10%). The resulting RF frequency may vary over time in various ways:sawtooth, square or sinusoidal, for example.

Instead of, or in addition to changing the RF frequency to reduce thenoding effect, it is also possible to superimpose a (preferably RF)frequency upon any DC bias voltage that is supplied to the multipolearrangement, even when the DC bias voltage is zero.

FIG. 2A shows another example of noding. The ion beam IB, which isproduced by an ion source 2 which in this embodiment is arranged insidean ion optical arrangement 1 having a wall 18, is shown to pass throughan opening in an exit electrode 14. The ion beam 113 shows manyoscillations between the ion source 2 and the exit electrode 14. Moreimportantly, the ion beam 113 fans out after the exit electrode 14 toproduce an ion beam fan IB″. This is caused by noding: ions havingdifferent masses leave the exit electrode at different angles. Inparticular, the exit ion beam IB″ is shown to consist of two parts: atop part containing a first type of ions (top first two lines) and abottom part containing a second type of ions. Thus, the exit angle inmass dependent, which is undesirable.

FIG. 2B shows an example where the noding effect is at least partiallysuppressed by using the present invention. The exit ion beam IB″ stillfans out but the exit angles are evenly distributed and are no longerion specific, that is, mass dependent.

FIG. 3A schematically shows a multipole arrangement of a collision cell1 according to the invention in a first operation mode, in which therods are used as an RF multipole. The collision cell 1 is shown to havea housing 18, in which the multipole arrangement is accommodated. Thecollision cell 1 is further shown to comprise an entrance electrode 12and an exit electrode 14, which comprises an exit opening 15. All threesegments 11A, 11B & 11C of each rod have the same DC voltage in thisfirst or RF operation mode, as in FIG. 3A. This DC voltage may or maynot be equal to zero (ground).

In the partially expanded FIG. 3C it can be seen that the ions do notfollow straight lines but have oscillating trajectories. It can also beseen that the ions fan out evenly at the exit opening 15. This is thesuppressed noding effect that may occur in the RF operation mode andwhich will later be discussed in more detail. The electrical field linesEFL are also schematically shown in FIG. 3C.

FIG. 3B schematically shows the same multipole arrangement as in FIG.3A, but where different DC voltages are applied to each of the sectionsof the rods, so as to provide an einzel lens. An RF voltage is notapplied in FIG. 3B. The trajectories of the ions (three differenttrajectories T are shown) depend on the entrance angles but no longer onthe substantially random parameters as in the RF operation mode shown inFIG. 6A. The DC voltages that may be used are, for example, between −1kV and −2 kV at a beam energy of 2 keV (high energy). It can be seenthat the einzel lens causes ions having different trajectories to passthrough the exit opening 15. The einzel lens can therefore be said tofocus the ions in the second or DC operation mode, in which the ions mayhave a high energy.

It is noted that according to an additional aspect of the invention, thecollision cell may be heated to reduce so-called memory effects. Thatis, by heating the collision cell to a temperature of, for example, 50°C., stray ions are less likely to remain on the electrodes (rods and/orvanes) and on the inner walls of the collision cell. It will beunderstood that stray ions which remain behind in an experiment maydetrimentally influence any further experiment. A suitable temperaturerange is 40° C. to 70° C., preferably 45° C. to 55° C. Heating acollision cell is preferably achieved using electric heating.

As mentioned above, a further additional aspect of the invention isoperating a collision cell in a pressurized mode and in an evacuated(that is, non-pressurized) mode. This requires that the collision cellcan be pressurized and depressurized rapidly. In particular, a pressurerelease mechanism is desired that is fast and effective.

FIG. 4 schematically shows a collision cell in which the invention maybe applied. The collision cell 1 is shown to comprise a housing 18 inwhich a multipole arrangement is accommodated. In the example shown, themultipole arrangement is a hexapole arrangement comprising six elongatepoles or rods 11 which constitute electrodes. A radio frequency (RF)voltage may be fed to opposite pairs of poles 11 to produce an RFelectric field. Ions can enter the collision cell through an entranceaperture 13 and leave the collision cell through an exit aperture 15.The RF field produced by the multipole arrangement focuses the ions onthe longitudinal axis of the arrangement. This is particularly relevantwhen a collision gas is present in the collision cell, as collisions maycause the ions to deviate from their path.

According to an aspect of the invention, therefore, valve mechanisms areprovided which are particularly suitable for use in a collision cellhaving a pressurized and an evacuated operation mode, such as, but notlimited to, the collision cell of the present invention.

FIGS. 5A & 5B show a mechanism 20 for adjusting the pumping crosssection of a collision cell housing 18 having rods 11. The mechanism 20is shown to comprise a door or flap 21 which is connected via a hinge 22to the housing 18 of the collision cell 1. The flap 21 can be operatedby an actuator 23 of which one end is connected to the flap 21 and theother end is connected to a support element 24 attached to the housing18.

The actuator 23 shown in FIGS. 5A & 5B is a Bourdon tube. A Bourdon tubecomprises a bent tube. The bending radius of the bent tube can bedecreased if the pressure difference between the inner part and theouter part of tube increases. To this end, a gas tube 25, which is alsoconnected to the support element 24, is connected with the actuator 23.In the embodiment shown, the gas flows from the gas tube 25 through achannel in the support element 24 into the actuator 23 when the gaspressure in the gas tube 25 is higher than the gas pressure surroundingthe actuator 23. By letting gas flow into the actuator, its bendingradius decreases (the actuator straightens) and the flap is opened.Conversely, the gas flows from the actuator 23 through the supportelement 24 into the gas tube 25 when the gas pressure in the gas tube 25is lower than in the actuator 23. By letting gas flow out of theactuator, its bending radius increases (the actuator curves) and theflap is closed.

Thus, by providing a pressure difference between the gas tube 25 and theair (or other gas) outside the actuator 23, the flap can be quicklyopened or closed, thus allowing the gas pressure in the interior of thecollision cell 1 to quickly assume the gas pressure on its outside.

It is noted that the collision cell 1 may be accommodated in anear-vacuum environment, while the gas tube may be connected with anenvironment under atmospheric pressure. The gas used for inflating theinflatable actuator may be air. As the interior volume of the actuator23 and the gas tube 25 may be small, only a small amount of air or othergas is needed to inflate the actuator. This air or other gas may beprovided by a gas reservoir or by a pump. Thus, a small pump or valvecan be sufficient to indirectly operate the relatively large flap.

By using a Bourdon tube or similar actuator, a fast and effectivepressure regulation of a collision cell can be achieved. However, aBourdon tube is not the only type of actuator that may be used in acollision cell or similar pressurized chamber, as will be furtherexplained with reference to FIG. 9.

FIG. 6A schematically shows an electrostatic opening mechanism used in acollision cell. The collision cell 1 is shown to comprise a housing 18in which rods 11 are accommodated. An ion beam IB can pass through thecollision cell 1, through openings in the front plate 12 and back plate14 respectively. In the embodiment shown, part of the wall of thehousing 18 is provided with through holes 16 which can be closed off bya movable foil. This foil is located in a spacing between the housing 18and a plate 19. Both the housing 18 and the plate 19 containelectrically conductive material and may both be made of metal, or atleast contain a metal layer or other conductive layer. The plate 19,which extends substantially parallel to the housing 18, may be flat butmay alternatively be curved to accommodate any curvature of the housing18.

In the embodiment shown, the foil comprises two layers: a conductivelayer 30 and an electrically insulating layer 31. A further electricallyinsulating layer 32 is attached to the plate 19. In an alternativeembodiment, the foil consists of three layers: the conductive layer 30and both insulating layers 31 & 32. Further layers may be added, as longas the foil remains sufficiently flexible. A suitable material for theinsulating layers 31 & 32 is Kapton, but other materials, for exampleother polyimides, may also be used. The conductive layer may be made ofcopper foil, for example.

As mentioned above, the flexible foil is located in the spacing betweenthe housing 18 and the plate 19. One edge of the foil may be attached tothe housing 18 while the opposite edge may be attached to the plate 19,such that the foil bridges the spacing. By applying DC voltages to theconductive layer, the position of the foils can be changed, as shown inFIG. 9A by the arrows which indicate the possible movement of thesubstantially S-shaped spacing-bridging portion of the foil.

Referring to FIG. 6B, the housing 18 will typically be connected toground (GND). The conductive plate 19 can be connected to a highvoltage, indicated by HV in FIG. 9B, thus creating a voltage differenceover the spacing between the housing 18 and the plate 19. If theconductive layer 30 is connected to a high voltage, then the foil willbe repelled by the plate 19 and attracted by the housing 18. As aconsequence, the foil will tend to move towards the housing and theS-shaped spacing bridging part will move to the right (see also FIG.9A). In other words, electrical forces F_(el) pulling the foil towardsthe housing cause a mechanical force F_(m) to the right in FIG. 9B. Thefoil will cover the through holes 16 and the interior of the collisioncell will be closed off.

Referring to FIG. 6C, the through holes 16 can be opened by connectingthe conductive layer 30 to ground instead of to the high voltage (HV).This will cause the foil to be repelled by the housing 18 and to beattracted by the plate 19, which in turn cause the S-shaped spacingbridging part to move to the left (see also FIG. 9A). In other words,electrical forces F_(el) pulling the foil towards the plate 19 cause amechanical force F_(m) to the left in FIG. 9C. The foil will no longercover the through holes 16 and the interior of the collision cell willbe open to the surrounding atmosphere.

As the movement of the foil is controlled by voltages, which can beswitched extremely quickly, and as the foil can have a very low mass,the movement of the foil can be very quick. Accordingly, the pressureinside the collision cell 1 can be adjusted very rapidly and switchingbetween a pressurized state and an evacuated state can be carried outalmost instantly.

The exemplary mass spectrometer 10 schematically shown in FIG. 7comprises a collision cell 1, which can be a collision cell as describedabove. The mass spectrometer 10 may further comprise a plasma source 1,such as an ICP (inductively coupled plasma) source for generating an ionbeam IB1. The mass spectrometer may further comprise a mass filter 3,such as a magnetic sector mass filter. In the magnetic sector massfilter, the ion beam IB1 is separated into partial beams IB2 havingdifferent m/z (mass versus charge) ratios, which partial beams can bedetected by the detector assembly 4, which may be a multiple detectorassembly. The mass spectrometer 10 may further comprise a pump forlowering the gas pressure in the collision cell 1, a valve associatedwith the pump, a voltage source 5 for supplying DC and AC (RF) voltagesto the collision cell 1, and a controller for controlling the variouscomponents of the mass spectrometer 10. The valve may comprise afoil-based valve and/or a Bourdon tube-based valve as described above.

Aspects of the invention comprise:

-   a) A multipole collision cell with variation of the number of    oscillations in RF mode in order to average mass dependent    trajectories and thus to counter the noding effect.-   b) A multipole collision cell in which an AC voltage is superimposed    upon a DC voltage, such as a multipole bias voltage, so as to    counter the noding effect.-   c) A multipole collision cell which can rapidly switched between a    first operation mode, in which the collision gas at least partly    mitigates the noding effect, and a second operation mode in which no    collision gas is used but which allows higher ion energies.-   d) Mechanisms for allowing a rapid switch between the first    operation mode and the second operation mode.    These aspects of the invention may be used in isolation or in    combination.

Although the invention has been described above mainly with reference toa collision cell or a collision/reaction cell, the invention is not solimited and may also be utilized in other ion optic arrangements, suchas mass filters and/or ion optic lens systems.

It will be understood by those skilled in the art that the invention isnot limited to the embodiments shown and that many additions and/ormodifications can be made without departing from the scope of theinvention as defined in the appending claims.

1. An ion optical arrangement for use in a mass spectrometer comprising:electrodes comprising a multipole arrangement defining an ion opticalaxis, and a voltage source for providing voltages to the electrodes toproduce electric fields, wherein the ion optical arrangement isconfigured for producing a radio frequency electric focusing field forfocusing ions on the ion optical axis, and wherein the radio frequencyelectric focusing field has a varying frequency so as to reduce any massdependence of ion trajectories through the ion optical arrangement. 2.The ion optical arrangement according to claim 1, wherein the varyingfrequency is a cyclically varying frequency.
 3. The ion opticalarrangement according to claim 1, wherein the varying frequency variesby at least 10%.
 4. The ion optical arrangement according to claim 1,wherein the voltage source is configured for supplying a voltage havinga varying frequency to the multipole arrangement so as to produce theradio frequency electric focusing field having a varying frequency. 5.The ion optical arrangement according to claim 1, wherein the multipolearrangement comprises at least four poles. 6.-13. (canceled)
 14. The ionoptical arrangement according to claim 1 claims, wherein the ion opticalarrangement comprises a mass filter.
 15. The ion optical arrangementaccording to claim 1, wherein the ion optical arrangement comprises acollision/reaction cell.
 16. A mass spectrometer comprising an ionoptical arrangement according to claim
 1. 17. The mass spectrometeraccording to claim 16, further comprising at least one ion source, suchas an inductively coupled plasma ion source, and at least one detectorarrangement, such as a multi-collector detector arrangement.
 18. Amethod of operating an ion optical arrangement for use in a massspectrometer, the ion optical arrangement comprising: electrodescomprising a multipole arrangement defining an ion optical axis, and avoltage source for providing voltages to the electrodes to produceelectric fields, wherein the ion optical arrangement is configured forproducing a radio frequency electric focusing field for focusing ions onthe ion optical axis, the method comprising varying the frequency of theradio frequency electric focusing field so as to reduce any massdependence of ion trajectories through the ion optical arrangement. 19.The method according to claim 18, wherein the varying frequency is acyclically varying frequency.
 20. The method according to claim 18,wherein the varying frequency varies by at least 10%. 21.-24. (canceled)25. A method according to claim 18, further comprising determining thenumber of oscillations of an ion beam passing through the ion opticalarrangement over the length of the ion optical arrangement, and varyingthe frequency of the radio frequency electric focusing field and/orsetting the frequency of the varying electric field superimposed uponthe static electric field such that the number of oscillations ischanged by at least one.
 26. (canceled)
 27. The ion optical arrangementaccording to claim 3, wherein the varying frequency varies by at least20%.
 28. The ion optical arrangement according to claim 5, wherein themultipole arrangement comprises at least six poles.
 29. The massspectrometer according to claim 17, further comprising a mass filter.30. The method according to claim 20, wherein the varying frequencyvaries by at least 20%.